Liquid crystal display

ABSTRACT

The present invention relates to a liquid crystal display provided with an electrostatic protection element and an object of the present invention is to provide the liquid crystal display provided with superior redundancy and at the same time a sufficient protection function against static electricity in which relatively low voltage generates for a long period of time. Electrostatic protection element sections  28  and  30  are provided with a first TFT  32  having a source electrode (S) and a drain electrode (D) where the source electrode (S) is connected to external output electrodes  16  and  18  and the drain electrode (D) is connected to common wirings  22  and  24 , a second TFT  38  having a conductor  42 , a source electrode (S), a drain electrode (D) and a gate electrode (G) where the conductor  42  is connected to the gate electrode (G) of the first TFT  32 , the source electrode (S) is connected to the external output electrodes  16  and  18 , the drain electrode (D) is connected to the conductor  42  and the gate electrode (G) is electrically floated, and a third TFT  40  having a source electrode (S), a drain electrode (D) and a gate electrode (G) where the source electrode (S) is connected to the common wirings  22  and  24 , the drain electrode (D) is connected to the conductor  42  and the gate electrode is electrically floated.

BACKGROUND OF THE INVENTION:

1. Field of the Invention

The present invention related to an active matrix type liquid crystaldisplay provided with a thin film transistor (hereinafter, referred toas TFT) as a switching element, and specifically relates to the liquidcrystal display provided with an electrostatic protection elementprotecting the TFT formed on a substrate on an array side and areasbetween bus lines from a destruction or a shortage due to staticelectricity.

2. Description of the Related Art

The active matrix type LCD is widely used in computers or equipment forthe use of OA (Office Automation) as a flat panel display providingsuperior picture quality. In this active matrix type LCD, a voltage isapplied electrode from both electrodes to a liquid crystal layer sealedbetween the substrate on the array side forming TFT and pixel electrodeand an opposing substrate forming common, thereby driving liquidcrystal.

A plurality of gate bus lines to which a scanning signal is sequentiallyinput to select a driving display pixel are formed in parallel to eachother on the substrate on the array side. Further, an insulation film isformed on the plurality of gate bus lines, and a plurality of data buslines in substantially orthogonal to the gate bus lines are formed on aninsulation film. Each area decided by the plurality of gate bus linesand the plurality of data bus lines formed in orthogonal to each otherin a matrix shape becomes a pixel area, and the TFT and the displayelectrode are formed in each pixel area. A gate electrode of the TFT isconnected to a predetermined gate bus line, a drain electrode isconnected to a predetermined data bus line, and a source electrode isconnected to the display electrode in the pixel area.

Incidentally, since the TFT for controlling the operation of liquidcrystal of the TFT-LCD, gate bus lines, data bus lines and the like areformed on the glass substrate which is an insulation material, the TFT,gate bus lines, data bus lines and the like are basically weak againststatic electricity. Therefore, if static electricity is generated on thesubstrate on the array side during a period from the substrate processon the array side constructing the TFT to the panel process sealingliquid crystal by attaching the substrate on the array side and theopposing substrate and a mounting a driver IC and the like, defects suchas a destruction of the TFT, a change in characteristics of the TFT, ashortage between each of the bus lines are generated, thereby resultingin a considerable reduction in fabrication yield of panels. Thus, areliable measure to protect the elements and the bus lines on thesubstrate on the array side from static electricity is required.

As a measure to protect the substrate on the array side from staticelectricity, for example, a method connecting all the bus lines to acommon electrode (short ring) to keep the same potential is known. Theshort ring is formed by materials for the data bus lines or the gate buslines when the data bus lines or the gate bus lines are formed. Thus,each bus line is electrically connected with the value of resistanceless than several k Ω. Therefore, even if a specific area on the panelis charged with electricity, the electric charges are instantlydispersed, thereby preventing the TFT in a display from a devicedestruction or a change in characteristics.

However, according to this method, since each bus line isshort-circuited to each other, an independent signal can not be appliedto each bus line. Therefore, a problem occurs, in which an arrayinspection (a TFT inspection) performing a characteristic test of theTFT of each pixel by detecting the amount of electric charges when theelectric charges are kept between the pixel electrode and the commonelectrode in the display panel can not be performed. Further, since theshort ring electrically connects the adjacent bus lines at lowresistance, the short ring is required to be removed either in the panelprocess or in the unit assembling process after the panel is completed.Thus, a problem exists in which a measure against static electricity isnot taken in the processes after the unit assembling process.

Accordingly, a method of arranging a resistive component between theshort ring and each bus line is conceived. FIG. 28 is a diagramdescribing the conventional technology connecting the resistivecomponent between the bus line and the short ring, which is disclosed inthe publication of Japanese Laid Open Patent Application No. 8-101397.FIG. 28 shows a part of a substrate surface on the array side and aresistance layer 400 is formed by patterning an ITO (indium tin oxide)formed on a gate metal or on a drain metal into a zigzag line at the endportion of a bus line 504. A tip of the zigzag-shape resistance layer400 is connected to a short ring 506. The array inspection is possibleaccording to this structure. Normally, this resistance layer 400 and theshort ring 506 are removed by disconnecting a scribe line SL shown bythe dotted line in the diagram in the panel scribe process whenassembling the panel.

However, according to this method, in order to obtain a higherresistance using ITO, an area is required to secure to lengthen thedistance of the zigzag-shape. Thus, a problem that an external size ofthe panel becomes large exists.

Besides the methods described above, a method for inserting anelectrostatic protection element such as a transistor and the likebetween the bus line and the short ring is conceived. For example, inthe publication of Japanese Laid Open Patent Application No. 61-79259, amethod of connecting the gate electrode and the source/drain electrodeby a capacitive coupling is shown.

FIGS. 29 a and 29 b are diagrams describing the conventional technologyshown in the publication of Japanese Laid Open Patent Application No.61-79259. FIG. 29 a shows a state of a part of the substrate on thearray side when viewing toward the substrate and FIG. 29 b shows a crosssection of the electrostatic protection element. As shown in FIG. 29 a,an electrostatic protection element 500 has a TFT structure arrangedbetween an external output electrode 504 at the end portion of a busline 502 and the short ring 506. The electrostatic protection element500 is formed by the same process as the TFT formed in the pixel area ona glass substrate 508. As shown in FIG. 29 b, a gate electrode 510 isformed on the glass substrate 508 and an operating semiconductor layer514 composed of, for example, amorphous silicon (hereinafter,abbreviated as a-Si) is formed on the gate electrode 510 via a gateinsulation film 512. A protection film 520 is formed on the operatingsemiconductor layer 514 and a source electrode 518 and a drain electrode516 are formed on both sides of the operating semiconductor layer 514sandwiching the protection film. The drain electrode 516 is connected tothe short ring 506 and the source electrode 518 is connected to theexternal output electrode 504. When viewing to the direction of thesubstrate surface, the gate electrode 510 has a plane overlapping withthe source/drain electrodes 518 and 516 and is connected with thesource/drain electrodes 518 and 516 by capacitive coupling. Therefore,when high voltage is generated due to static electricity between thesource/drain electrodes 518 and 516, since the potential of the gateelectrode 510 becomes the middle of the potential difference generatedbetween the source/drain electrodes 518 and 516, a channel is created atthe operating semiconductor layer 514, thereby releasing the electricload due to static electricity from the bus line 502.

However, since the structure of this electrostatic protection element500 has a single structuring element, the redundancy is poor. In otherwords, since high voltage due to static electricity is received by onlyone TFT, the electrostatic protection element 500 is easily destroyedand when the area between the bus line 502 and the short ring 506 isinsulated due to the destruction, the possibility of the TFT in thepixel area to be exposed to static electricity increases. Further, evenif irregularities due to static electricity do not occur, if theelectrostatic protection element 500 is short-circuited due to somereason, a TFT test can not be performed.

Next, the electrostatic protection circuit having more redundancy thanthe structure shown in FIGS. 29 a and 29 b which is disclosed in thepublication of Japanese Laid Open Patent Application No. 10-303431 isdescribed with reference to FIG. 30. The source electrode (S) of thefirst TFT 530 which is the electrostatic protection element is connectedto an external output electrode 502 of the bus line, and the drainelectrode (D) on the other side is connected to the short ring 506. Thegate electrode (G) of the first TFT 530 is connected to a conductor 536which is electrically floated from both the external output electrode502 and the short ring 506. On the other hand, the source electrode (S)and the gate electrode (G) of the second TFT 532 are connected to theexternal output electrode 502 of the bus line and the m drain electrode(D) on the other side is connected to the conductor 536. Further, thedrain electrode (D) of the third TFT 534 is connected to the conductor536 and the source electrode (S) and the gate electrode (G) on the otherside are connected to the short ring 506. When positive high voltage isgenerated in the bus line with respect to the short ring 506 due tostatic electricity, high voltage is applied to the gate electrode (G) ofthe second TFT 532 and a channel is formed, thereby rapidly increasingthe conductivity. On the other hand, since the gate electrode (G) of thethird TFT 534 is connected to the short ring 506, a channel is notformed and the conductivity remains to be very small. This difference inconductivity is very large, and in consequence, the potential of theconductor 536 is substantially equal to the potential of the bus line.As a result, a channel is formed by applying the voltage between the busline and the short ring 506 at the gate electrode of the first TFT 530which is the electrostatic protection element and the electric chargecan be released. It will be noted that the second and the third TFT's532 and 534 do not basically run the current and are used only tocontrol the gate potential of the first TFT 530.

In this manner, in the above electrostatic protection circuit, since thegate electrodes of the second and the third TFT's 532 and 534 areconnected to the external output electrode 502 of the bus line or theshort ring 506, the potential difference between the external outputelectrode 502 and the short ring 506 is instantly liquidated. However,when the voltage generated by static electricity reduces as the timepasses, the potential of the conductor 536 also reduces and theconductivity of the first TFT 530 reduces. Thus, when the voltage isrelatively low (˜ several volts) due to static electricity, theefficiency of releasing the electric charges is reduced.

Also, based on the previous fabrication experiences, obstacles due tostatic electricity are known to be occurred by sharp pulse-like staticelectricity at extremely high voltage for a short period of time andstatic electricity continuously applied to each element for a longperiod of time even if the voltage is relatively low. Therefore,although the electrostatic protection circuit described in thepublication of Japanese Laid open Patent Application No. 10-303431 canbe expected to be effective in the former case, little result isexpected in the latter case as the path for the current to escape is cutoff when the voltage is reduced to a certain extent. Further, accordingto the electrostatic protection circuit described in the abovepublication, since the current due to static electricity all flows inthe first TFT, the redundancy is poor and the load is exceedinglyincreased, therefore the possibility of the first TFT to be destroyedexists. Furthermore, since the gate electrode (G) of the second TFT 532is directly connected with the external output electrode 502 of the busline and the gate electrode (G) of the third TFT 534 is directlyconnected with the short ring 506, the redundancy against shortage isreduced.

As still another conventional electrostatic protection circuit, there isa structure shown in FIG. 31, which is described in the publication ofJapanese Laid Open Patent Application No. 7-60875. This is anelectrostatic protection circuit connecting between the bus line 504 andthe short ring 506 via a resistive component by a two-way transistorusing non-linear elements 402 and 404. Besides the two-way transistor, anon-linear element such as a shot-key diode, which can be a resistivecomponent, may be also used. Since the resistive component by thenon-linear element has a sufficient high resistive component so as notto affect the operation of each bus line, the resistive component by thenon-linear element can be remained after the panel is completed.Further, as to static electricity, since some current which can disperseelectric charges flows, the resistive component by the non-linearelement functions as an anti-electrostatic element.

Although in the method arranging the high resistive component by thenon-linear element such as the two-way transistor, the high resistivecomponent can be formed in a relatively small area, problems occur withrespect to controlling the current since the structure of the devicebecomes complex and moreover the resistive component is altered by anexternal charges (for example, static electricity) owing to thenon-linear element. Further, since the high resistive component can notbe formed outside the ensured area for operation of operatingsemiconductor film of the transistor such as the area adjacent to an endface of glass, a problem of not being able to make the size of the panellarge against a mother glass exists.

Accordingly, although the short ring is required to be removed in thepanel process or in the unit assembly process after the panel iscompleted according to the conventional liquid crystal display, aproblem exists in which a measure against static electricity can not betaken in the processes after the short ring is removed.

Further, in the method arranging the zigzag pattern using the ITO, aproblem exists in which if the length of the zigzag pattern is long, theexternal size of the panel becomes large.

Furthermore, the conventional liquid crystal display has problems inwhich the electrostatic protection element (circuit) for preventing adevice destruction due to static electricity is poor in redundancy, thearea between the bus line and the short ring is easily short-circuitedor the electrostatic protection element does not function as aprotection circuit against the static electricity generating mrelatively low voltage for a long period of time.

Also, if the non-linear element such as the two-way transistor is usedas the high resistive component, the structure of the device becomescomplex and the aspect of controlling the current is disadvantageous aswell. Further, since the non-linear element can not be formed adjacentto the end face of the glass, a problem of not being able to make thesize of the panel large against the mother glass exists.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a liquid crystaldisplay provided with an electrostatic protection circuit superior inredundancy.

Another object of the present invention is to provide a liquid crystaldisplay provided with a sufficient protection function against staticelectricity generating relatively low voltage for a long period of time.

Further object of the present invention is to provide a liquid crystaldisplay in which measures for static electricity can be taken until thelast stage of the substrate assembly process.

Furthermore object of the present invention is to provide a liquidcrystal display in which an electrostatic protection element sectiondoes not affect the size of a panel.

Another object of the present invention is to provide a liquid crystaldisplay having the electrostatic protection element section which issimple in structure of the device and not disadvantageous in an aspectof controlling the current.

Above objects are achieved by an active matrix type liquid crystaldisplay comprising a switching element formed for each of a plurality ofpixels decided by a plurality of bus lines and a short ring connected tothe plurality of bus lines, and an electrostatic protection elementportion formed between each of the plurality of bus lines and the shortring, wherein the electrostatic protection element portion comprises athin film transistor having a source or a drain electrode connected tothe bus line and the drain or the source electrode connected to theshort ring, a first resistor connecting a gate electrode of the thinfilm transistor to the bus line, and a second resistor connecting thegate electrode of the thin film transistor to the short ring.

In the liquid crystal display described above, the second resistor maybe a common resistor connecting the gate electrodes of the plurality ofthin film transistor to the short ring.

Further, the above object are achieved by an active matrix type liquidcrystal display comprising a switching element formed for each of aplurality of pixels decided by a plurality of bus lines and anelectrostatic protection element portion formed between the adjacent buslines, wherein the electrostatic protection element portion comprises athin film transistor having a source or a drain electrode connected toone of the adjacent bus lines and the drain or the source electrodeconnected to the other of the bus lines, a first resistor connecting agate electrode of the thin film transistor to one of the bus lines, anda second resistor connecting the gate electrode of the thin filmtransistor to the other of the bus lines.

Furthermore, above objects are achieved by an active matrix type liquidcrystal display comprising a switching element formed for each of aplurality of pixels decided by a plurality of bus lines, a short ringconnected to the plurality of bus lines, and a electrostatic protectionelement portion formed between each of the plurality of bus lines andthe short ring, wherein the electrostatic protection element portioncomprises a first thin film transistor having a source or a drainelectrode connected to the bus line and the drain or the sourceelectrode connected to the short ring, a conductive material connectedto a gate electrode of the first thin film transistor, a second thinfilm transistor having a source or a drain electrode connected to thebus line, the drain or a source electrode connected to the conductivematerial, and a gate electrode electrically floated, and a third thinfilm transistor having a source or a drain electrode connected to theshort ring, the drain or the source electrode connected to theconductive material, and a gate electrode electrically floated.

In the liquid crystal display described above, the third thin filmtransistor may be a common transistor connecting the gate electrodes ofthe plurality of first thin film transistors to the short ring.

Further, above objects are achieved by an active matrix type liquidcrystal display comprising a switching element formed for each of aplurality of pixels decided by a plurality of bus lines, and anelectrostatic protection element portion formed between the adjacent buslines, wherein the electrostatic protection element portion comprises afirst thin film transistor having a source or a drain electrodesconnected to one of the adjacent bus lines and the drain or the sourceelectrode connected to the other of the bus lines, a conductive materialconnected to a gate electrode of the first thin film transistor, asecond thin film transistor having a source or a drain electrodeconnected to one of the bus lines, the drain or the source electrodeconnected to the conductive material, and a gate electrode electricallyfloated, and a third thin film transistor having a source or a drainelectrode connected to the other of the bus lines, the drain or thesource electrode connected to the conductive material, and a gateelectrode electrically floated.

In the liquid crystal display of the present invention above, the gateelectrode of the first transistor can be connected to the conductivematerial via capacitor. Also, a channel length of at least one of thesecond and the third thin film transistors can be shorter than a channellength of the first thin film transistor.

In the conventional electrostatic protection circuit short-circuitingthe gate electrodes (G) of the second and the third TFT's 532 and 534with the bus line 502 and the short ring 506 respectively as shown inFIG. 30, the current does not flow in the second and the third TFT's 532and 534 in reality and the electrostatic protection circuit is used onlyfor controlling the gate potential of the first TFT 530. On the otherhand, the first and the second resistors or the second and the thirdTFT's of the present invention show a two-way conductivity between thebus line and the short ring, thereby enabling the current to flow. Thus,the first and the second resistors or the second and the third TFT'shave a function of preliminarily releasing the charge due to staticelectricity before the first TFT for primarily running the currentsufficiently conducts. In other words, since the current preliminarilyflows in the second and the third TFT's, the load on the first TFT canbe reduced and the redundancy of the electrostatic protection circuitcan be improved.

Further, the gate electrode of the first TFT of the present invention isconnected with the bus line and the short ring via capacitors and thepotential of the gate electrode gently varies for a time required tocharge and discharge these capacitors. Therefore, according to thestructure of the present invention, gentle static electricity cansufficiently be dealt with. When the capacitor is inserted between thegate electrode of the first TFT and a common conductor between thesecond and the third TFT, a reaction becomes gentle as a whole furtherand the efficiency as the electrostatic protection element is improved.

Furthermore, the structure shown in FIG. 30 has more number of elementsthan the structure shown in FIGS. 29 a and 29 b and the redundancy isimproved. However, for example, if the gate electrode (G) and the drainelectrode (D) of the second TFT 532 are short-circuited and at the sametime the gate electrode (G) and the drain electrode (D) of the first TFT530 are short-circuited, the function as the electrostatic protectioncircuit is lost. Similarly, when the gate electrode (G) and the drainelectrode (D) of the third TFT 534 are short-circuited and at the sametime the gate electrode (G) and the drain electrode (D) of the first TFT530 are short-circuited, or when the gate electrode (G) and the drainelectrode (D) of the second TFT 532 are short-circuited and at the sametime the gate electrode (G) and the drain electrode (D) of the third TFT530 are short-circuited, the function as the electrostatic protectioncircuit is also lost. In other words, according to the circuit shown inFIG. 30, if the elements in the circuit are short-circuited at twoplaces as described above, a defect occurs.

On the contrary, for example, describing this embodiment with referenceto FIG. 3, in the structure according to the present invention, if thegate electrode (G) and the source electrode (S) of the second TFT 38 areshort-circuited and the gate electrode (G) and the drain electrode (D)of the second TFT 38 are also short-circuited and at the same time thegate electrode (G) and the drain electrode (D) of the first TFT 32 areshort-circuited, the function as the electrostatic protection circuit islost. Similarly, when the gate electrode (G) and the source electrode(S) of the third TFT 40 are short-circuited, and the gate electrode (G)and the drain electrode (D) of the third TFT 40 are also short-circuitedand at the same time the gate electrode (G) and the drain electrode (D)of the first TFT 32 are short-circuited, or when the gate electrode (G)and the source electrode (S) of the second TFT 38 are short-circuited,the gate electrode (G) and the drain electrode (D) of the second TFT 38are also short-circuited, the gate electrode (G) and the drain electrode(D) of the second TFT 38 are also short-circuited, the gate electrode(G) and the source electrode (S) of the third TFT 40 are alsoshort-circuited, and at the same time the gate electrode (G) and thedrain electrode (D) of the third TFT 40 are short-circuited, thefunction as the electrostatic protection circuit is lost. In otherwords, according to the specific circuit of the present invention shownin FIG. 3, when the elements in the circuit short-circuit at more thanthree places, then the function stops for the first time as theelectrostatic protection circuit. Thus, since the gate in theelectrostatic protection circuit according to the present invention isin a floating state, the redundancy for a shortage of the structuringelements is also superior.

Further, above objects are achieved by an active matrix type liquidcrystal display comprising a switching element formed for each of aplurality of pixels decided by a plurality of bus lines, a short ringconnected to the plurality of bus lines, and an electrostatic protectionelement portion formed between each of the plurality of bus lines andthe short ring, wherein the electrostatic protection element portioncomprises a plurality of metal layers, an insulating layer formed on theplurality of metal layers, a contact hole formed by opening theinsulating layer on the plurality of metal layers, and a connectinglayer electrically connecting between the metal layers via the contacthole.

Furthermore, above objects are achieved by an active matrix type liquidcrystal display comprising, a switching element formed for each of aplurality of pixels decided by a plurality of bus lines, and anelectrostatic protection element portion formed between the adjacent buslines, wherein the electrostatic protection element portion comprises aplurality of metal layers, an insulating layer formed on the pluralityof metal layers, a contact hole formed by opening the insulating layeron the plurality of metal layers, and a connecting layer electricallyconnecting between the metal layers via the contact hole.

According to the present invention, the contact holes are formed on theprotection film on the gate bus line or the data (drain) bus line andthe short ring and each of the bus lines are electrically connected viathe contact holes. A contact resistance generated between differentmetals (for example, Ti and ITO) in this structure can obtain the ohmiccontact by selecting materials and the resistance value of the resistivecomponent can also be controlled by the number or the size of thecontact holes, or by the subsequent treatment processes for theunderlying metal. The metal contact is certainly not limited to theohmic contact and a resistive device having a non-linear characteristiccan be arranged by the shot-key connection.

Since the anti-electrostatic element formed according to the presentinvention is easy to control the resistance (current control) and thestructure is simple as well, a stable resistive component can be held.Further, since an arbitrary resistive component can be formed by themethod previously described, the array inspection can be possible and asufficient protective function against static electricity can also beheld by constructing the resistive component.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 1 is a diagram showing a schematic structure of a liquid crystaldisplay according to a first embodiment of the present invention.

FIGS. 2 a and 2 b are diagrams showing a circuit structure and anoperation of an electrostatic protection element section according tothe first embodiment of the present invention.

FIG. 3 is a diagram showing a circuit structure of an electrostaticprotection element which is a characteristic component of a liquidcrystal display according to a second embodiment of the presentinvention.

FIGS. 4 a, 4 b and 4 c are diagrams showing a structure of theelectrostatic protection circuit according to the second embodiment ofthe present invention.

FIG. 5 is a diagram showing an example of a variation of theelectrostatic protection circuit of the liquid crystal display accordingto the second embodiment of the present invention.

FIG. 6 is a diagram showing a state of an electrostatic protectioncircuit of a liquid crystal display according to a third embodiment ofthe present invention when viewing toward a substrate.

FIG. 7 is a diagram showing a circuit structure of a electrostaticprotection element section which is the characteristic component of aliquid crystal display according to a fourth embodiment of the presentinvention.

FIGS. 8 a, 8 b and 8 c are diagrams showing a structure of theelectrostatic protection circuit according to the fourth embodiment ofthe present invention.

FIG. 9 is a diagram showing an example of a variation of the structureof the electrostatic protection circuit according to the fourthembodiment of the present invention.

FIG. 10 is a diagram showing an example of a variation of theelectrostatic protection circuit according to the fourth embodiment ofthe present invention.

FIG. 11 is a diagram showing an other example of the variation of theelectrostatic protection circuit according to the fourth embodiment ofthe present invention.

FIG. 12 is a diagram showing a circuit of an electrostatic protectionelement section of a liquid crystal display according to a fifthembodiment of the present invention.

FIG. 13 is a diagram showing an example of a variation of theelectrostatic protection circuit of the liquid crystal display accordingto the fifth embodiment of the present invention.

FIG. 14 is a diagram showing a circuit of an electrostatic protectionelement section of a liquid crystal display according to a sixthembodiment of the present invention.

FIG. 15 is a diagram showing a structure of the electrostatic protectioncircuit of the liquid crystal display according to the sixth embodimentof the present invention.

FIG. 16 is a diagram showing an example of a variation of theelectrostatic protection circuit of the liquid crystal display accordingto the sixth embodiment of the present invention.

FIG. 17 is a diagram showing a structure of an example of a variation ofthe electrostatic protection circuit of the liquid crystal displayaccording to the sixth embodiment of the present invention.

FIG. 18 is a diagram showing a circuit of an electrostatic protectionelement section of a liquid crystal display according to a seventhembodiment of the present invention.

FIG. 19 is a diagram showing a structure of the electrostatic protectioncircuit of the liquid crystal display according to the seventhembodiment of the present invention.

FIG. 20 is a diagram showing an example of a variation of theelectrostatic protection circuit of the liquid crystal display accordingto the seventh embodiment of the present invention.

FIG. 21 is a diagram showing a structure of an example of a variation ofthe electrostatic protection circuit of the liquid crystal displayaccording to the seventh embodiment of the present invention.

FIG. 22 is a diagram showing a structure of an example of a variation ofthe electrostatic protection circuit of the liquid crystal displayaccording to the first through the seventh embodiments of the presentinvention.

FIGS. 23 a and 23 b are diagrams showing a structure of an electrostaticprotection circuit of a liquid crystal display according to an eighthembodiment of the present invention.

FIGS. 24 a and 24 b are diagrams showing a structure of an example of avariation of the electrostatic protection circuit of the liquid crystaldisplay according to the eighth embodiment of the present invention.

FIG. 25 is a diagram showing a fabrication process of the electrostaticprotection circuit of the liquid crystal display according to the eighthembodiment of the present invention.

FIG. 26 is a diagram showing a structure of other example of thevariation of the electrostatic protection circuit of the liquid crystaldisplay according to the eighth embodiment of the present invention.

FIG. 27 is a diagram showing a structure of an example of an applicationof the electrostatic protection circuit of the liquid crystal displayaccording to the eighth embodiment of the present invention.

FIG. 28 is a diagram showing a structure of the electrostatic protectioncircuit of a conventional liquid crystal display.

FIGS. 29 a and 29 b are diagrams showing a structure of theelectrostatic protection circuit of the conventional liquid crystaldisplay.

FIG. 30 is a diagram showing a structure of the electrostatic protectioncircuit of the conventional liquid crystal display.

FIG. 31 is a diagram showing a structure of the electrostatic protectioncircuit of the conventional liquid crystal display.

DESCRIPTION OF THE PREFERRED EMBODIMENTS:

A liquid crystal display according to a first embodiment of the presentinvention is described with reference to FIG. 1, FIG. 2 a and FIG. 2 b.First, a schematic structure of the liquid crystal display according tothis embodiment is described with reference to FIG. 1. FIG. 1 shows apart of a substrate 1 on an array side of this liquid crystal displayviewing toward a substrate surface. It will be noted that inside of apixel area shows an equivalent circuit for driving liquid crystal. Onthe substrate 1 on the array side, a plurality of gate bus lines 2extending in the horizontal direction in the diagram are formed inparallel in the vertical direction. Furthermore, although omitted in thediagram, an insulation film is formed on the plurality of gate bus lines2, and a plurality of data bus lines 4 are formed on the insulation filmin substantially orthogonal to the gate bus lines 2. Each area decidedby the gate bus lines 2 and the data bus lines 4 which crossorthogonally to each other in a matrix shape becomes a pixel area, and aTFT 6 and a display electrode 8 are formed in each pixel area. A gateelectrode of the TFT 6 is connected to a predetermined gate bus line 2,a drain electrode is connected to a predetermined data bus line 4, and asource electrode is connected to the display electrode 8 in the pixelall area. A dotted line 14 in the diagram shows an end portion of anopposing substrate. On the opposing substrate side, common electrode 12is formed. Liquid crystal 10 is sealed between the substrate 1 on thearray side and the opposing substrate.

The TFT 6 in which the gate electrode is connected to the gate bus line2 is set to “On” state by a scanning signal outputted to thepredetermined gate bus line 2, and the voltage based on a gradationsignal outputted to the data bus line 4 is applied to a pixel electrode8. On the other hand, a predetermined voltage is also applied to thecommon electrode 12 on the opposing substrate side and the liquidcrystal between the pixel electrode 8 and the common electrode 12 isdriven by the voltage applied to the pixel electrode 8 and the commonelectrode 12.

An external output electrode 16 is formed at the end portion of eachgate bus line 2 and an external output electrode 18 is also formed atthe end portion of each data bus line 4. A short ring 20 which is acomponent of an electrostatic protection circuit is formed at theexternal surrounding of the external output electrodes 16 and 18. Theshort ring 20 has a common wiring 22 on the gate bus line side and acommon wiring 24 on the data bus line side. An electrostatic protectionelement section 28 which is a component of the electrostatic protectioncircuit is formed between the common wiring 22 on the gate bus line sideand the external output electrode 16 on each of the gate bus line 2. Onthe other hand, an electrostatic protection element section 30 which isa component of the electrostatic protection circuit is formed betweenthe common wiring 24 on the data bus line side and the external outputelectrode 18 on each data bus line 4.

Next, a circuit structure and operation of the electrostatic protectionelement sections 28 and 30 according to this embodiment is describedwith reference to FIG. 2 a. It will be noted that since the structureand operation of the electrostatic protection element section 28 and theelectrostatic protection element section 30 are the same, hereinafter,the electrostatic protection element section 28 is described as anexample. The electrostatic protection element section 28 has a TFT 32, afirst resistor 34 and a second resistor 36. A source electrode (S) ofthe TFT 32 which is an electrostatic protection element is connected tothe external output electrode 16 on the gate bus line 2. On the otherhand, a drain electrode (D) is connected to the common wiring 22. A gateelectrode (G) of the TFT 32 is connected to the external outputelectrode 16 by the first resistor 34 and at the same time, the gateelectrode (G) of the TFT 32 is connected to the common wiring 22 by thesecond resistor 36. When a positive high voltage generates to the busline with respect to the common wiring 22 due to static electricity, thevoltage with the value which divides the high voltage generated due tothe static electricity with the first resistor 34 and the secondresistor 36 is applied to the gate electrode (G) of the TFT 32. As aresult, since the conductivity of the TFT 32 rapidly increases, electriccharges due to static electricity are released via the TFT 32. At thistime, the electric charges are released not only via the TFT 32 but alsovia the first and the second resistors 34 and 36. The current flowing inthe TFT 32 is relieved in comparison with the case where the TFT is asingle unit as shown in FIGS. 29 a and 29 b and is further superior inredundancy as the electrostatic protection element to a protectioncircuit shown in FIG. 30. Therefore, the liquid crystal display havingthe electrostatic protection circuit which is not easily destroyed bystatic electricity and in which TFT tests can also be sufficientlyperformed can be fabricated.

Next, a circuit structure of the electrostatic protection elementsections 28(=30) according to another example is described withreference to FIG. 2 b. The electrostatic protection element section 28has the TFT 32, the first resistor 34, the second resistor 36, aconductor 42, and a capacitor 100.

The gate electrode (G) of the TFT 32 is connected to the conductor 42which is electrically insulated from either the external outputelectrode 16 of the bus line 2 or the common wiring 22. The firstresistor 34 is connected between the external output electrode 16 andthe conductor 42. The second resistor 36 is connected between the commonwiring 22 and the conductor 42.

The capacitor 100 is formed between the conductor 42 and the gateelectrode (G) of the TFT 32. When static electricity is generated, theoperation of the TFT 32 becomes gentle due to the capacitor 100.Further, since the capacitor 100 is added, the redundancy againstdefects due to shortage is also improved.

Next, a liquid crystal display according to a second embodiment of thepresent invention is described with reference to FIG. 3 through FIG. 5.Since a schematic structure of this liquid crystal display is similar toFIG. 1 used in the first embodiment, description is omitted, and acircuit structure of the electrostatic protection element sections 28and 30 which are characteristic components is described with referenceto FIG. 3. The electrostatic protection element section 28 has the firstthrough the third TFT's 32, 38 and 40, and a conductor 42. The sourceelectrode (S) of the first TFT 32 which is the electrostatic protectionelement is connected to the external output electrode 16 of the bus line2, and the drain electrode (D) on the other side is connected to thecommon wiring 22. The gate electrode (G) of the first TFT 32 isconnected to the conductor 42 which is electrically insulated fromeither the external output electrode 16 of the bus line 2 or the commonwiring 22. On the other hand, the source electrode (S) of the second TFT38 is connected to the external output electrode 16 and the drainelectrode (D) on the other side is connected to the conductor 42.Further, the drain electrode (D) of the third TFT 40 is connected to theconductor 42 and the source electrode (S) on the other side is connectedto the common wiring 22. Furthermore, the gate electrode (G) of thesecond and the third TFT's 38 and 40 are not connected to any patternsand are isolated. When a positive high voltage is generated to the busline with respect to the common wiring 22 by static electricity, a highvoltage internally divided by each parasitic capacitor (C2 _(gs), C2_(gd), C3 _(gs), C3 _(gd)) is applied to the gate electrodes (G) of thesecond and the third TFT's 38 and 40 and a channel is formed in thesecond and third TFT's 38 and 40. As a result, the current flows throughthe second and the third TFT's 38 and 40, and the potential of theconductor 42 also increases. Hence, a channel is formed in the first TFT32 and the conductivity increases, thereby releasing the electriccharges due to static electricity. According to this embodiment, sincethe current preliminarily flows to the second and the third TFT's 38 and40 in this manner, a load on the first TFT 32 is reduced and theredundancy of the electrostatic protection circuit can be improved.Further, the gate electrode (G) of the first TFT 32 is connected to theexternal output electrodes 16 and 18 via the capacitors and to thecommon wirings 22 and 24 of the short ring 20, and the potential of thegate electrode (G) gently varies for only the time required to chargeand discharge these capacitors. Therefore, according to the structure inthis embodiment, gentle static electricity can be sufficiently dealtwith.

Since the electric charges are released through a plurality of paths inthis manner, in comparison with the case in the past where there is asingle TFT, the load to the first TFT is relieved. Further, since theredundancy as the electrostatic protection element increases, the liquidcrystal display having the electrostatic protection circuit which is noteasily destroyed by static electricity and in which TFT tests can alsobe performed sufficiently can be fabricated.

Next, a structure of the electrostatic protection circuit according tothis embodiment is described with reference to FIGS. 4 a, 4 b and 4 c.FIG. 4 a shows a single electrostatic protection circuit on thesubstrate 1 on the array side viewing toward the substrate surface. FIG.4 b shows a cross section cut at a line A-A′ in FIG. 4 a. FIG. 4 c showsa cross section cut at a line B-B′ in FIG. 4 a.

In FIG. 4 a, the electrostatic protection element section 28 (or 30,hereinafter, description omitted) is formed between the common wiring 22(or 24, hereinafter, description omitted) extending vertically in theleft side of the diagram and the external output electrode 16 (or 18,hereinafter, description omitted). As shown in FIGS. 4 b and 4 c, whenthe gate bus line 2 and the gate electrode of the TFT 6 (refer toFIG. 1) in the pixel area are formed on a glass substrate 50, the gateelectrodes (G) of the first through the third TFT's 32, 38 and 40 arealso simultaneously formed. The gate electrodes (G) of the second andthe third TFT's 38 and 40 are formed electrically floated from otherwiring structures. A gate insulation film 52 is formed on the gateelectrodes (G) and the glass substrate 50. An operating semiconductorlayer 44 made of a-Si is patterned individually on the gate insulationfilm 52 formed on each gate electrode (G) of the first through the thirdTFT's 32, 38 and 40. The data (drain) bus line 4 and a source/drainelectrode patterned when simultaneously forming the external outputelectrode 16 are formed on both sides sandwiching each operatingsemiconductor layer 44. End portions of each source/drain electrode lieover each operating semiconductor layer 44. When viewing toward thesubstrate surface, an area, where the end portion of each source/drainelectrode and the lower layer gate electrode (G) overlap, is formed.Further, the short ring 22 is also simultaneously formed when the databus line 4 is formed. A passivation film 54 is formed on a whole surfaceof the element formation area.

A contact hole 56 is formed by removing the passivation film 54 onsubstantially the center of the source/drain electrode between thesecond and the third TFT's 38 and 40. Similarly, a contact hole 58 isformed by removing the gate insulation film 52 and the passivation film54 on an end portion of the gate electrode of the first TFT 32. Aportion substantially the center of the source/drain electrode betweenthe second and the third TFT's 38 and 40 and the gate electrode of thefirst TFT 32 are connected by an ITO layer 43 constructing a part of theconductor via the two contact holes 56 and 58. In this example, an ITOlayer 42 which is a component of the conductor 42 is simultaneouslyformed when patterning the ITO as a transparent electrode for forming adisplay electrode in each pixel area.

In the structure shown in FIGS. 4 a, 4 b and 4 c, both of the externaloutput electrodes 16 and 18 and the common wirings 22 and 24 of theshort ring 20 are formed simultaneously with the formation of the databus line 4 and by the same formation material as the data bus line 4.However, this is not essential. For example, as shown in FIG. 5, theexternal output electrodes 16 and 18 and the short rings 22 and 24 mayalso be formed simultaneously with the formation of the gate bus line 2and by the same metal layer as the gate bus line 2. FIG. 5 shows a statein which an electrostatic protection circuit on the substrate 1 on thearray side is viewed toward the substrate surface. As shown in FIG. 5, asource electrode 70 of the first TFT 32 to be connected with theexternal output electrodes 16 and 18 is connected at an ITO layer 72which is a layer when forming the display electrode, via a contact hole74 formed on an portion of the source electrode 70 and a contact hole 76formed on the external output electrodes 16 and 18. Similarly, a sourceelectrode 60 of the second TFT 38 to be connected with the externaloutput electrodes 16 and 18 is connected by an ITO layer 62 which is alayer when forming the display electrode, via a contact hole 64 formedon an end portion of the source electrode 60 and a contact hole 66formed on the external output electrodes 16 and 18. Further, in the samemanner, a drain electrode 80 of the first TFT 32 to be connected withthe common wirings 22 and 24 of the short ring 20 and a source electrode90 of the third TFT 40 are respectively connected by ITO layers 82 and92 which are layers when forming the display electrode, via contactholes 84 and 94 formed on end portions of the drain electrode 80 and thesource electrode 90 and contact holes 86 and 96 formed on the commonwirings 22 and 24 respectively.

Next, a liquid crystal display according to a third embodiment of thepresent invention is described with reference to FIG. 6. FIG. 6 shows astate of the electrostatic protection circuit on the substrate 1 on thearray side when viewing toward the substrate surface. The liquid crystaldisplay according to this embodiment also has distinctivecharacteristics in the electrostatic protection circuit and since othercomponents are the same as the components described in the firstembodiment with reference to FIG. 1, descriptions for the othercomponents are omitted. Further, in the electrostatic protection elementsection, components having the similar function and operation to thefirst and the second embodiments are also referred by the same codes anddescriptions are omitted. The electrostatic protection circuit accordingto this embodiment has a distinctive characteristic in forming theelectrostatic protection element sections 28 and 30 described in thesecond embodiment with reference to FIGS. 4 a, 4 b and 4 c between theadjacent bus lines, thereby not forming the short ring 20. In otherwords, the source electrode of the first TFT 32 is connected to one ofthe two adjacent bus lines 2 (or 4, hereinafter, description omitted),and the drain electrode is connected to the other of the two adjacentbus lines 2. Further, the source electrode of the second TFT 38 isconnected to one of the two adjacent bus lines 2 and the sourceelectrode of the third TFT 40 is connected to the other of the twoadjacent bus lines 2. Except for the differences in the structure above,the similar effects to the second embodiment can be obtained by theelectrostatic protection circuit according to this embodiment.

Next, a liquid crystal display according to a fourth embodiment of thepresent invention is described with reference to FIG. 7 through FIG. 11.Since the schematic structure of this liquid crystal display is similarto FIG. 1 used in the first embodiment, description is omitted, and thecircuit structure of the electrostatic protection element portions 28and 30 which are characteristic components is described with referenceto FIG. 7. However, components exhibiting the similar functions andoperations to the structures shown in FIGS. 3, 4 a, 4 b and 4 c arereferred by the same codes and descriptions are omitted.

The electrostatic protection element portion 28 according to thisembodiment, as is the case in the second embodiment, has the firstthrough the third TFT's 32, 38 and 40 and the conductor 42. Thedifference from the second embodiment is to have a capacitor 100. Thecapacitor 100 is formed between the conductor 42 and the gate electrode(G) of the first TFT 32. When static electricity is generated, theoperation of the first TFT 32 becomes gentle due to the capacitor 100 incomparison with the second and the third TFT's 38 and 40. Therefore, inthe case of static electricity generating sharp pulse-like variations involtage, the current first flows to the second and the third TFT's 38and 40, thereby protecting the first TFT 32. Further, in the case ofstatic electricity generating a gentle increase in voltage, followingthe second and the third TFT's 38 and 40, the first TFT 32 operates andcontributes for releasing the electric charges. According to thisembodiment, since the current preliminarily flows to the second and thethird TFT's 38 and 40 in this manner, the load on the first TFT 32 isreduced and the redundancy of the electrostatic protection circuit canbe improved. Furthermore, since the gate electrode (G) of the first TFT32 is connected with the external output electrodes 16 and 18 and thecommon wirings 22 and 24 of the short ring 20 via the capacitor, thepotential of the gate electrode (G) varies gently for a time required tocharge and discharge these capacitors. Therefore, according to thestructure in this embodiment, gentle static electricity can besufficiently dealt with. Also, according to this embodiment, since thecapacitor 100 is inserted between the gate electrode (G) of the firstTFT 32 and the common conductor 42 between the second and the thirdTFT's 38 and 40, even if the difference in potential between theexternal output electrodes 16 and 18 and the common wirings 22 and 24 ofthe short ring 20 is reduced, a state of continuity can be kept stilllonger for a time required to charge and discharge the capacitor 100,and the efficiency in charge release can be further improved. Further,since the capacitor 100 is added, the redundancy against defects due toshortage is also improved. In this embodiment, since the electriccharges are also released through a plurality of paths, the redundancyas the electrostatic protection element increases in comparison with thecase in the past where there is a single TFT, therefore the protectioncircuit which is not easily destroyed by static electricity can beformed.

Next, a structure of the electrostatic protection circuit according tothis embodiment is described with reference to FIGS. 8 a, 8 b and 8 c.FIG. 8 a shows a state of a single electrostatic protection circuit onthe substrate 1 on the array side when viewing toward the substrate.FIG. 8 b shows a cross section cut at a line A-A′ in FIG. 8 a. FIG. 8 cshows a cross section cut at a line B-B′ in FIG. 8 a.

In FIG. 8 a, the electrostatic protection element portion 28 is formedbetween the common wiring 22 extending vertically in the left side ofthe diagram and the external output electrode 16. As shown in FIGS. 8 band 8 c, when the gate bus line 2 and the gate electrode of the TFT 6(refer to FIG. 1) in the pixel area are formed, the gate electrodes (G)of the first through the third TFT's 32, 38 and 40 are alsosimultaneously formed on the glass substrate 50. The gate electrodes (G)of the second and the third TFT's 38 and 40 are formed electricallyfloated from other wiring structures. The gate insulation film 52 isformed on the gate electrodes (G) and the glass substrate 50. Theoperating semiconductor layer 44 made of a-Si is patterned individuallyon the gate insulation film 52 formed on each gate electrode (G) of thefirst through the third TFT's. The source/drain electrode issimultaneously patterned when the data (drain) bus line 4 and externaloutput electrode 16 are formed on both sides sandwiching each of theoperating semiconductor layer 44. The end portions of each source/drainelectrode are formed lying over each operating semiconductor layer 44.Further, the short ring 22 is also simultaneously formed when the databus line 4 is formed. The passivation film 54 is formed on the wholesurface of the element formation area.

The source/drain electrode between the second and the third TFT's 38 and40 functions as the conductor 42 and also forms the capacitor 100between the source/drain electrode and the gate electrode (G) of thefirst TFT 32 extending to the lower part of the conductor 42.

In the structure shown in FIGS. 8 a, 8 b and 8 c, the external outputelectrodes 16 and 18 and the short rings 22 and 24 are formedsimultaneously with the formation of the data bus line 4 and by the sameformation material as the data bus line 4. However, this is notessential. For example, as shown in FIG. 9, the external outputelectrodes 16 and 18 and the short rings 22 and 24 may also be formedsimultaneously by the same metal layer as the gate bus line 2 when thegate bus line 2 is formed. The structure shown in FIG. 9 can be obtainedby changing the connections of the wirings in the same manner asdescribed with reference to FIG. 5.

Next, an example of a variation of the electrostatic protection circuitaccording to this embodiment is described with reference to FIG. 10 andFIG. 11. In the first and the second embodiments and this embodiment,the short ring 20 and the electrostatic protection element sections 28and 30 are arranged outside the external output electrodes 16 and 18 onthe substrate on the array side. Therefore, the short ring 20 and theelectrostatic protection element sections 28 and 30 can be removed by abeveling process after panel scribing. On the other hand, if the shortring 20 is arranged inside the external output electrodes 16 and 18, theglass can be efficiently utilized without waste by reducing an area forscribing on the glass substrate. In this case, the short ring 20 and theelectrostatic protection element sections 28 and 30 remain in the liquidcrystal panel even after panel scribing and each of the bus lines 2 and4 is short-circuited via the electrostatic protection circuit. However,the resistance is so large that an interference between each bus linecan be ignored and the quality of a product is not at all affected. Aposition to form the short ring 20 can be considered in the same manneras in all embodiments to be described hereinafter.

FIG. 10 shows an example of the electrostatic protection circuitstructure forming the common wiring 24 of the short ring 20 inside theexternal output electrode 18 of the data bus line 4. The electrostaticprotection element section 30 is formed between the common wiring 24extending vertically in the diagram and the pixel area (opposite side ofthe external output electrode 18 with respect to the common wiring 24)which is not identified in the diagram. The gate electrodes (G) of thefirst through the third TFT's 32, 38 and 40 are simultaneously formed onthe glass substrate 50 when the gate bus line 2 and the gate electrodeof the TFT 6 (refer to FIG. 1) in the pixel area are formed. The gateelectrodes (G) of the second and the third TFT's 38 and 40 are formedelectrically floated from the other wiring structures. Further, thecommon wiring 24 is also formed simultaneously when the gate bus line 2is formed. The drain electrode (D) of the first TFT 32 and the drainelectrode (D) of the third TFT 40 are connected to the common wiring 24via a contact hole portion 77.

The source/drain electrode between the second and the third TFT's 38 and40 functions as the conductor 42 and also forms the capacitor 100between the source/drain electrode and the gate electrode (G) of thefirst TFT 32 extending to the lower part of the conductor 42.

Further, in this example, the channel lengths of the second and thethird TFT's 38 and 40 are formed shorter than the channel length of thefirst TFT 32. Thus, when static electricity producing extremely sharppulse-like voltage is generated in the data line 4, the second or thethird TFT's 38 or 40 are first destroyed before the first TFT 32 isdestroyed, thereby protecting the first TFT 32. Therefore, If even ifany one of the second and the third TFT's 38 and 40 is destroyed, thedata bus line 4 and the common wiring 24 do not directly short-circuitand do not provide an obstacle in sequential processes including TFTtests. Furthermore, in this example, the channel lengths of the secondand third TFT's 38 and 40 are the same and are at the same time equal toapproximately half the channel length of the first TFT 32. Also, thechannel widths of the second and the third TFT 38 and 40 are the sameand are at the same time approximately the same width as the channelwidth of the first TFT 32. Therefore, the conductivity of the first TFT32 and the conductivity of the second and the third TFT's 38 and 40 whenobserved serially are approximately the same, and the current can bedivided into approximately halves by the first TFT 32 and the second andthe third TFT's 38 and 40.

FIG. 11 shows an example of the electrostatic protection circuitstructure forming the common wiring 22 of the short ring 20 inside theexternal output electrode 16 of the gate bus line 2. The electrostaticprotection element section 28 is formed between the common wiring 22extending vertically in the diagram and the pixel area (opposite side ofthe external output electrode 16 with to the common wiring 22) which isnot identified in the diagram. The gate electrodes (G) of the firstthrough the third TFT's 32, 38 and 40 are simultaneously formed on theglass substrate 50 when the gate bus line 2 and the gate electrode ofthe TFT 6 (refer to FIG. 1) in the pixel area are formed. The gateelectrodes (G) of the second and the third TFT's 30 and 40 are formedelectrically floated from the other wiring structures.

The source/drain electrodes of the first through the third TFT's 32, 38and 40 and the common wiring 22 are formed simultaneously with theformation of the data bus line and by the same formation material as thedata bus line. The source electrode (S) of the first TFT and the sourceelectrode (S) of the second TFT 38 are connected to the gate bus line 2via contact hole portions 78 and 79 respectively.

The source/drain electrode between the second and the third TFT's 38 and40 functions as the conductor 42 and also forms the capacitor 100between the source/drain electrode and the gate electrode (G) of thefirst TFT extending to the lower part of the conductor 42.

Further, in this example, as is the case shown in FIG. 10, the channellengths of the second and third TFT's 38 and 40 are the same and are atthe same time equal to approximately half the channel length of thefirst TFT 32. Also, the channel widths of the second and the third TFT38 and 40 are the same and are at the same time approximately the samewidth as the channel width of the first TFT 32. Therefore, theconductivity of the first TFT 32 and the conductivity of the second andthe third TFT's 38 and 40 when observed serially are approximately thesame, and the current can be divided into approximately halves by thefirst TFT 32 and the second and the third TFT's 38 and 40.

Next, a liquid crystal display according to a fifth embodiment of thepresent invention is described with reference to FIG. 12 and FIG. 13.While in the first through the fourth embodiments described above, a setof electrostatic protection element sections are respectively formed ineach bus line, the liquid crystal display holding the elements formed inthe electrostatic protection element section in common as much aspossible and reducing the number of whole elements is shown in thisembodiment. When a generation ratio of component defects, an areaoccupied by the elements and the like are considered, reducing thenumber of structuring elements as much as possible is more desirable.

A circuit of the electrostatic protection element section according tothis embodiment is shown in FIG. 12. As shown in FIG. 12, inelectrostatic protection element sections 28-1 and 28-2 (or 30-1 and30-2), TFT's 32-1 and 32-2 and the first resistors 34-1 and 34-2 areformed at each of the external output electrodes 16-1 and 16-2 (or 19-1and 18-2). The second resistor 36 is not formed in each of the elementsections 28-1 and 28-2. Instead, the conductor 42, to which theelectrodes (G) of the first TFT's 32-1 and 32-2 are connected, and thecommon wirings 22 and 24 are connected at a single common resistor 37 asthe second resistor. By providing the common resistor 37, the number ofelements structuring the electrostatic protection element sections canbe reduced to ¾ in comparison with the first through the fourthembodiments.

For example, if a positive high voltage is generated in the bus line ofthe external output electrode 16 against the common wiring 22 due tostatic electricity, the voltage value obtained by dividing the highvoltage generated due to static electricity by the first resistor 34-1and the common resistor 37 is applied to the gate electrodes (G) ofTFT's 32-1 and 32-2. As a result, since the conductivity of TFT's 32-1and 32-2 rapidly increase, the electric charges due to staticelectricity is released via TFT's 32-1 and 32-2. At this time, since theelectric charges are released not only via TFT's 32-1 and 32-2 but alsovia the first resistors 34-1 and 34-2 and the common resister 37 and thecurrent flowing through the TFT 32-1 is relieved, the redundancy as theelectrostatic protection element increases, thereby realizing theelectrostatic protection circuit which is not easily destroyed by staticelectricity.

Next, an example of a variation of this embodiment is described withreference to FIG. 13. In order to reduce the number of elementsstructuring the electrostatic protection circuits as much as possible,the structure shown in FIG. 12 is further proceeded. The structure shownin FIG. 13 has a distinctive characteristic in using a single commonresistor 37 in common among the electrostatic protection elementsections 28-1 through 28-n (or 30-1 through 30-n) of more than n (n isan integer of more than 3) bus lines.

In the electrostatic protection element sections 28-1 through 28-nprovided at each of the external output electrodes 16-1 through 16-n,TFT's 32-1 through 32-n and the first resistor 34-1 through 34-n arerespectively formed. The second resistor 36 is not formed in each of theelement sections 21-1 through 28-n. Instead, the conductor 42 to whichthe gate electrodes (G) of the first TFT's 32-1 through 32-n areconnected and the common wirings 22 and 24 are connected by the commonresistor 37 as the single second resistor in place of individual secondresistors.

If the common resistor 37 is used in place of individual secondresistors in the electrostatic protection element sections 28 and 30 ofall the bus lines, the number of structuring elements per a bus line canbe approximately 2 and the number of elements used in the electrostaticprotection circuits according to the first embodiment can be reduced toapproximately half.

Next, a liquid crystal according to a sixth embodiment of the presentinvention is described with reference to FIG. 14 through FIG. 17. Whilein the liquid crystal display according to the second embodiment above,a set of electrostatic protection element sections are respectivelyformed in each bus line, in this embodiment as is the case in the fifthembodiment, the liquid crystal display holding the elements formed inthe electrostatic protection element section in common as much aspossible and reducing the number of whole elements is shown.

A circuit of the electrostatic protection element section according tothis embodiment is shown in FIG. 14. As shown in FIG. 14, inelectrostatic protection element sections 28-1 and 28-2 (or 30-1 and30-2), the first TFT's 32-1 and 32-2 and the second TFT's 38-1 and 38-2are formed at each of the external output electrodes 16-2 and 16-2 (or18-1 and 18-2). The third TFT 40 is not formed in each of the elementsections 28-1 and 28-2. Instead, the conductor 42 to which theelectrodes (G) of the first TFT's 32-1 and 32-2 are connected and thecommon wirings 22 and 24 are connected at a common TFT 41 as the singlethird TFT in place of the third individual TFT. By providing the commonTFT 41, the number of elements structuring the electrostatic protectionelement sections can be reduced to ¾ in comparison with the firstthrough the fourth embodiments.

For example, if a positive high voltage is generated in the bus line ofthe external output electrode 16-1 against the common wiring 22 due tostatic electricity, the high voltage interior-divided by each parasiticcapacitor (C2 _(gs), C2 _(gd), Cc_(gs), Cc_(gd)) is applied to thesecond TFT 38-1 and the gate electrode (G) of the common TFT 41 andchannels are formed in the second TFT 38-1 and the common TFT 41. As aresult, the current flows through the second TFT 38-1 and the common TFT41, and the potential of the conductor 42 also increases. Hence, achannel is formed in the first TFT 32-1 and the conductivity increases,thereby releasing the electric charges due to static electricity. Sincethe electric charges are also released through a plurality of paths inthis case, the amount of the electric charges flowing in the first TFT32 is relieved in comparison with the case in the past having a singleTFT. Therefore, the redundancy as the electrostatic protection elementincreases and the protection circuit which can not be easily destroyedby static electricity can be formed.

Next, the structure of the electrostatic protection circuit according tothis embodiment is described with reference to FIG. 15. FIG. 15 shows astate of a single electrostatic protection circuit on the substrate 1 onthe array side when viewing toward the substrate surface. In FIG. 15,the electrostatic protection element sections 28-1 and 28-2 are formedbetween the common wiring 22 extending vertically on the left side ofthe diagram and the external output electrodes 16-1 and 16-2.

In this example, the conductor 42 extends vertically in the diagram andis connected to the first TFT 32-1 of the electrostatic protectionelement section 28-1 side by the ITO layer 43 via contact holes 56-1 and58-1. Further, the conductor 42 is connected to the first TFT 32-2 ofthe electrostatic protection element section 28-2 side by the ITO layer43 via contact holes 56-2 and 58-2.

The operating semiconductor layer 44 made of a-Si is patterned on thegate insulation film on the gate electrode (G) of the common TFT 41. Thedrain electrode (D) of the common TFT 41 pulled out from substantiallythe center portion of the conductor 42 is connected at both sidessandwiching the operating semiconductor layer 44. The source electrodeof the common TFT 41 is connected to the common wirings 22 and 24. Endportions of the source/drain electrode of the common TFT 41 lie over theoperating semiconductor layer 44 and areas where end portions of eachsource/drain electrode and the under-layer gate electrodes (G) overlapare formed. The conductor 42, the external output electrodes 16-1 and16-2 and the common wirings 22 and 24 are simultaneously formed when thedata bus line 4 is formed.

Next, an example of a variation of this embodiment is described withreference to FIG. 16. In order to reduce the number of elementsstructuring the electrostatic protection circuits as many as possible,the structure shown in FIG. 15 is further proceeded. The structure shownin FIG. 16 has a distinctive characteristic in using a single common TFT41 among the electrostatic protection element sections 28-1 through 28-n(or 30-1 through 30-n) of more than n (n is an integer of more than 3)bus lines.

In the electrostatic protection element sections 28-1 through 28-nprovided at each of the external output electrodes 16-1 through 16-n,the first TFT's 32-1 through 32-n and the second TFT's 38-1 through 38-nare respectively formed. The third TFT 40 is not formed in each of theelement sections 28-1 through 28-n. Instead, the conductor 42 to whichthe gate electrodes (G) of the first TFT's 32-1 through 32-n areconnected and the common wirings 22 and 24 are connected at the commonTFT 41 as the single third TFT in place of individual third TFT.

If the common TFT 41 is used in place of the third TFT 40 in theelectrostatic protection element sections 28 and 30 of all the buslines, the number of structuring elements per a bus line can beapproximately 2 and the number of elements used in the electrostaticprotection circuits according to the second embodiment can be reduced toapproximately half.

Next, the structure of the electrostatic protection circuit according tothis embodiment is described with reference to FIG. 17. FIG. 17 shows astate of a single electrostatic protection circuit on the substrate 1 onthe array side when viewing toward the substrate surface. In FIG. 17,the electrostatic protection element sections 28-1 and 28-n are formedbetween the common wiring 22 extending vertically in the left side onthe diagram and the external output electrodes 16-1 and 16-n.

In this example, the conductor 42 extends vertically in the diagram andis connected to the gate electrodes of a plurality of the first TFT's32-1 through 32-n. Further, the second TFT's 38-1 through 38-n areconnected to the conductor 42 by the ITO layer 43 via contact holes.Since the structure of the common TFT 41 is the same as the structuredescribed with reference to FIG. 15, description is omitted. The drainelectrode of the common TFT 41 is connected to the conductor 42 by theITO layer 43 via contact holes and the source electrode is connected tothe common wirings 22 and 24.

Next, a liquid crystal display according to a seventh embodiment of thepresent invention is described with reference to FIG. 18 through FIG.21. While a set of electrostatic protection element sections arerespectively formed in each bus line in the liquid crystal displayaccording to the third embodiment above, in this embodiment as is thecase in the fifth and sixth embodiments, the liquid crystal displayholding the elements formed in the electrostatic protection elementsection in common as much as possible and reducing the number of wholeelements is shown.

The circuit of the electrostatic protection element m section accordingto this embodiment is shown in FIG. 18. As shown in FIG. 18, capacitors100-1 and 100-2 are formed in each of the electrostatic protectionelement sections 28-1 and 28-2. The third TFT 40 is not formed in eachof the element sections 28-1 and 28-2. Instead, the conductor 42 towhich the electrodes (G) of the first TFT's 32-1 and 32-2 are connectedand the common wirings 22 and 24 are connected at a common TFT 41 as thesingle third TFT in place of the third individual TFT. By providing thecommon TFT 41, the number of elements structuring the electrostaticprotection element sections can be reduced to ¾ in comparison with thefirst through the fourth embodiments.

According to this embodiment, by having the capacitor 100, operations ofthe first TFT's 32-1 and 32-2 when static electricity is generated alsobecome gentle in comparison with operations of the second TFT's 38-1 and38-2 and the common TFT 41. Therefore, in the case of static electricitygenerating sharp pulse-like variations in voltage, the current firstflows to the second TFT's 38-1 and 38-2 and the common TFT 41, therebyprotecting the first TFT's 32-1 and 32-2. Further, in the case of staticelectricity gently increasing in voltage, the first TFT's 32-1 and 32-2operate following the second TFT's 38-1 and 38-2 and the common TFT 41and contribute to the release of the electric charge. According to thisembodiment, since the current preliminarily flows to the second TFT's38-1 and 38-2 and the common TFT 41, the load on the first TFT's 32-1and 32-2 is reduced, thereby increasing the redundancy of theelectrostatic protection circuit. Further, the gate electrodes of thefirst TFT's 32-1 and 32-2 are respectively connected with the externaloutput electrodes 16-1, 18-1, 16-2 and 18-2 and the common wirings 22and 24 of the short ring 20 via the capacitors, and the potential of thegate electrode (G) gently varies for a time required to charge anddischarge these capacitors. Therefore, according to the structure ofthis embodiment, even gentle static electricity can be sufficientlydealt with. Furthermore, in this embodiment, since the capacitors 100-1and 100-2 are inserted between the gate electrodes (G) of the firstTFT's 32-1 and 32-2 and the common conductor 42 between the second TFT's38-1 and 38-2 and the common TFT 41, even if the potential differencebetween the external output electrodes 16 and 18 and the common wirings22 and 24 of the short ring 20 is reduced, continuity state can bemaintained still longer for a time required to charge and discharge thecapacitors 100-1 and 100-2, thereby further improving the efficiency ofcharge release. Also, by adding the capacitors 100-1 and 100-2, theredundancy against defects due to shortage is improved. Since theelectric charges are also released through a plurality of paths in thisembodiment, the redundancy as the electrostatic protection elementincreases in comparison with the case in the past having a single TFTand destruction of elements due to static electricity does not easilyoccur.

Next, the structure of the electrostatic protection circuit according tothis embodiment is described with reference to FIG. 19. FIG. 19 shows astate of a single electrostatic protection circuit on the substrate 1 onthe array side when viewing toward the substrate. The first distinctivecharacteristic of the structure shown in FIG. 19, with respect to thestructure shown in FIG. 15 is that the capacitors 100-1 and 100-2 areformed by positioning the gate electrodes of the first TFT's 32-1 and32-2 at the lower layer of the conductor 42 via the insulation film.Since other structures are the same as the structure shown in FIG. 15,description is omitted.

Next, an example of a variation of this embodiment is described withreference to FIG. 20 and FIG. 21. In order to reduce the number ofelements structuring the electrostatic protection circuits as many aspossible, the structure shown in FIG. 18 is further proceeded. Thestructures shown in FIGS. 20 and 21 have a distinctive characteristic inusing a single common TFT 41 among the electrostatic protection elementsections 28-1 through 28-n (or 30-1 through 30-n) of more than n (n isan integer of more than 3) bus lines. The distinctive characteristic ofthe circuit structure and element structure shown in FIGS. 20 and 21,with respect to the circuit structure and element structure shown inFIGS. 16 and 17, is that the capacitors 100-1 through 100-n are formedby positioning the gate electrodes of the first TFT's 32-1 through 32-nat the lower layer of the conductor 42 via the insulation film. Sinceother structures are the same as the structures shown in FIGS. 16 and17, description is omitted.

In the fabrication process of the TFT for the substrate 1 on the arrayside where the electrostatic protection circuit according to the firstthrough the seventh embodiments described above is formed, the qualityof the panel may be judged by an open/short inspection (O/S inspection),not by a TFT test, for simply detecting a disconnection/shortage of thebus line. In this case, in order to detect an interlayer shortage, thecommon wiring 22 of the short ring 20 on the gate bus line 2 side andthe common wiring 24 on the data bus line 4 side are required to beelectrically separated by a high resistive component. Accordingly, thestructure shown in FIG. 22 can be taken as an example. In FIG. 22, forexample, an interlayer separation portion 23 having the similarstructure to the electrostatic protection element portions 28 and 30described with reference to FIG. 2 through FIG. 11 in the first throughthe fourth embodiments is formed at an intersection of the common wiring22 and the common wiring 24.

Further, as shown in FIG. 22, by connecting either the common wiring 22or 24 (the common wirings 22 in FIG. 22) of the short ring 20 to, forexample, the common electrode 12 on the opposing substrate side or to aconnecting terminal 25 connected to ground, the TFT's and the bus linescan also be protected even more certainly from obstacles due to staticelectricity.

Next, a liquid crystal display according to an eighth embodiment of thepresent invention is described. To begin with, a fabrication process ofthe substrate on the array side for a TFT-LCD used in this embodiment isbriefly described. First, a gate metal is deposited and patterned on thesubstrate on the array side, and the gate bus line and the gateelectrode of the TFT in each pixel area are formed. Second, the gateinsulation film is formed on the whole surface and an a-Si layer to bean operating semiconductor film of the TFT and an insulation film forforming a channel protection film are deposited in this order on thegate insulation film. Third, by a back exposure using the bus line andthe gate electrode as a mask and an exposure using an ordinary mask toelectrically separate the a-Si layer from the pixel area, the aboveinsulation film is patterned and the channel protection film is formed.Fourth, an n⁺ layer to be an ohmic contact layer, the drain/sourceelectrodes and a drain metal (for example, Ti (titanium)) layer to formthe data bus line are formed in this order on the whole surface. Fifth,the n⁺ layer and the drain metal layer are patterned and thedrain/source electrodes and the data bus line are formed. Sixth, thepassivation film (for example, SiN film (silicon nitride film) is formedand then patterned, and a contact hole is formed at a predeterminedposition on the passivation film. Seventh, by depositing the ITO on thewhole surface and then patterning, the pixel electrode is formed. In theabove process, the exposure process is included in the first, third,fifth, sixth and seventh processes, resulting in a five-mask processusing five masks in total.

The electrostatic protection circuit of this liquid crystal displayformed including the above processes is described in detail withreference to FIGS. 23 a through 27. It will be noted that, in thisembodiment, the structuring elements having the same function andoperation as in the first through the seventh embodiments are designatedby the same codes.

FIG. 23 a shows a state of the substrate on the array side when viewingtoward the substrate. FIG. 23 b shows a cross section cut at a line A-A′in FIG. 23 a. FIGS. 23 a and 23 b show a state of the external outputelectrode 18 pulled out of the data bus line 4 (not shown) on thesubstrate 1 which is a glass substrate on the array side and formed. Theelectrostatic protection element section 30 is formed at a tip of theexternal output electrode 18, and the external output electrode 18 andthe common wiring 24 of the short ring 20 are connected via theelectrostatic protection element section 30. Although omitted in thediagram, the structures of the gate bus line 2 and its external outputelectrode 16 are similar to the structures above.

As shown in FIG. 23 b, the gate insulation film 52 according to thesecond process above is formed on the substrate 1 on the array side. Thedrain metal layer according to the fourth process is patterned and theexternal output electrode 18 and the common wiring 24 are formed on thegate insulation film 52. Further, a metal layer 200 which patterns thedrain metal layer constructing a part of the electrostatic protectionelement section 30 is formed on the opposite side of the external outputelectrode 18 and the common wiring 24. The passivation film 54 isembedded between both end portions of the opposing metal layer 200 andthe end portions are electrically separated. Contact holes 98 which openthe passivation film 54 are respectively formed on both end portions ofthe opposing metal layer 200. The ITO layer 43 which is a conductivefilm deposited in the seventh process is patterned on the interior wallsof the two contact holes and between the contact holes, and the twoopposing metal layers 200 are electrically connected by the ITO layer43. In this case, the lower layer drain metal (Ti) and the upper layermetal (ITO) form an ohmic connection and a resistive component variesdepending on the size of the contact hole. When Ti is used for the lowerlayer metal, a heat treatment (for example, approximately 180° C.through 215° C.) is performed before depositing the ITO and when thediameter of the contact hole 98 is φ=4 μm, the resistive component to beformed is 7 to 8 Ω. Since the contact hole 98 is formed in the sixthprocess above and the ITO film is also formed in the seventh process,the electrostatic protection circuit can be formed without changing theconventional fabrication processes at all.

FIGS. 24 a and 24 b show examples of variation of this embodiment whichserially connects a plurality of contact holes 98 in order to make theelectrostatic protection element section 30 to have a high resist. InFIG. 24 a, a plurality of island-like metal layers 202 are furtherformed between the two opposing metal layers 200 where the tips arearranged on the opposing sides of the external output electrode 18 andthe common wiring 24. The contact holes 98 are formed on the passivationfilm 54 on both end portions of the plurality of metal layers 202 linedup linearly. The adjacent metal layers 200 and 202 are electricallyconnected by the ITO layer 43 via the contact holes 98.

In the structure shown in FIG. 24 b, electrically floated island-likemetal layers 204 are arranged adjacent to each opposing end portion ofthe metal layers 200 and 202 lined up linearly, and the contact holes 98are formed on both end portions of the metal layers 204. Each opposingend portion of the metal layers 200 and 202 is connected at theconnection layer of the ITO layer 43 via the metal layer 204 and thecontact holes 98. By arranging the electrostatic protection elementsection 30 in a zigzag line in this manner, the distance between thecommon wiring 24 and the external output electrode 18 can be reduced.

When reading the electric charges charged between the pixel electrodeand the common electrode by an array inspection device using anintegration circuit, the resistance value of more than 100 k Ω isdesired as an isolation resistance. Therefore, if the number of thecontact holes 98 are more than 14 by adopting the structure shown inFIGS. 24 a and 24 b, the electrostatic protection circuit which does notaffect the array inspection can be realized. Thus, according to thisembodiment, the electrostatic protection circuit having an arbitraryvalue of resistive component can be formed by connecting a plurality ofsteps of the resistors via the contact holes.

Next, an example of a variation which makes the lower layer metal as amulti-layer structure in the electrostatic protection element sectionaccording to this embodiment is described with reference to FIG. 25.FIG. 25 shows cross sections of the electrostatic protection elementsection in the formation process. A column (A) shows the gate bus lineside and a column (B) shows the data bus line side. Further, a row (a)through a row (e) show treatments in each process. First, in (a) of FIG.25, when the gate bus line and the gate electrode of the TFT are formedon the substrate 1 on the array side which is the glass substrate, ametal layer 200 g of the electrostatic protection element section 28 onthe gate bus line 2 side is simultaneously formed by gate metal. Whenforming the metal layer 200 g, the common wiring 22 of the short ring 20can also be simultaneously formed by gate metal. Next, the gateinsulation film 52 is formed on the whole surface by using, for example,SiN (silicon nitride).

Next, as shown in (b) of FIG. 25, when forming the data bus line 4 andthe drain/source electrodes of the TFT, a metal layer 200 d of theelectrostatic protection element section 30 on the data bus line 4 sideis simultaneously formed by using drain metal. The drain metal layer isconstructed by Ti/Al/Ti in order from the lower layer. Further, thecommon wiring 24 of the short ring 20 can also be formed by drain metalsimultaneously when forming the metal layer 200 d. Next, the passivationfilm 54 is formed on the whole surface.

Next, as shown in (c) of FIG. 25, the contact hole 98 is formed byopening the passivation film 54 on the metal layers on the metal layers200 g and 200 d. Further, as shown in (d) of FIG. 25, the contact hole98 where the upper portion of the metal layer 200 g exposes is formed byetching the gate insulation film 52 on the metal layer 200 g. In theprocess which collectively etches the passivation film 54 and the gateinsulation film 52, the top drain metal layer, the Ti layer, functionsas an etching stopper during the etching of the gate insulation film 42.At this time, if the thickness of the top drain metal layer Ti is thin,the underlying Al layer may be exposed.

Next, as shown in (e) of FIG. 25, the ITO layer 43 is formed bypatterning the ITO when forming the display electrode so that theadjacent metal layers 200, 202 and the like are electrically connectedvia the contact hole 98. At this time, since an ITO layer 43 a and theAl layer of the metal layer 200 d form a shot-key connection and thering-shaped Ti layer-remained in the contact hole 98 and an ITO layer 43b form an ohmic connection, an overall contact resistance can beincreased. for example, when the drain metal is Ti(20 nm)/Al(75nm)/Ti(20 nm), the contact resistance per a contact hole on the metallayer 200 d is equal to 35 to 36 k Ω and if 3 or 4 of the metal layers200 d are serially connected, a state possible for the array inspectioncan be obtained.

It will be noted that by varying the temperature of the heat treatmentunder the condition that the metal layer is exposed at the bottom of thecontact hole 98 and it is before forming the ITO layer 43, the contactresistance of the metal/ITO can be varied. When an element having muchhigher resistance is required, the baking temperature may be increased.

The resistance value of the resistive component formed in this mannercan be more than 10 MΩ. Even if the scanning signal, picture signal orthe like is applied to each bus line after the panel is completed, theadjacent bus lines can not be affected due to this high resistivecomponent. Therefore, these high resistive components can be remained inthe panel after the panel is completed. Thus, obstacles due to staticelectricity in the unit assembly process after the panel is completedcan be prevented, therefore the liquid crystal display can be fabricatedat a much higher yield and the reliability of the display can beimproved.

Although this embodiment described that the resistive components withthe arbitrary resistance value can be arranged by lining up a pluralityof the contact holes 98 linearly between each of the bus lines 2 and 4and the short ring 20 (the common wiring 22 and 24), this embodiment isnot limited to this and as shown in FIG. 26, the structure according tothis embodiment can be formed between the adjacent gate bus lines 2 orthe adjacent data bus lines 4. In this case, the electrostaticprotection circuit can also be remained in the panel after the panel iscompleted by connecting the areas between the contact holes arranged onthe metal layers 200, 202 and the like using the ITO layer and byforming sufficiently high resistive elements. The electrostaticprotection element portion according to this embodiment can certainly beformed without changing the fabrication process not only between theadjacent bus lines but also at an arbitrary position requiring the highresistive component.

Further, in the TFT fabrication process, the quality of the panel may bejudged by the open/short inspection (O/S inspection) for simplydetecting a disconnection/shortage of the bus line without the use ofthe array inspection. In this case, in order to detect an interlayershortage, the common wiring 22 of the short ring 20 on the gate bus line2 side and the common wiring 24 on the data bus line 4 side are requiredto be separated by the high resistive component. Accordingly, thestructure shown in FIG. 27 can be taken as an example. In the areaindicated by the dotted lines 120 in FIG. 27, a connecting state of thecommon wiring 22 of the short ring 20 and the common wiring 24 is shown.As shown in FIG. 27, by connecting a contact hole 121 where the endportion of the common wiring 22 formed by patterning the gate metallayer exposes and a contact hole 122 where the end portion of the commonwiring 22 formed by patterning the drain metal layer exposes by the ITOlayer 43, the high resistive portion can easily be formed at the endportion of connection. In the formation of the high resistive portion inthe contact hole 122, the resistance value can arbitrarily be adjustedby employing the method described in (d) and (e) of FIG. 25 above.

It should be noted that in the embodiment above, although the siliconnitride film is used as the insulation film, a silicon oxide film (SiO₂film) can certainly be used. Also, in the embodiment above, although theITO is used for the connection layer between the contact holes 98, thisembodiment is not limited to this and other materials relatively high inresistance value may certainly be used. Further, although the layeredstructure of Ti/Al/Ti is used as the drain metal, molybdenum (Mo),tungsten (W), tantalum (Ta) and their alloy, or their nitride oxide canbe used instead of Ti for the upper metal layer, and copper (Cu), Alalloy, Cu alloy and the like can be used instead of Al for the middlelayer.

It should be noted that the each of the structures described in FIGS. 23through 27 in the embodiment above can apply to the interlayerseparation portion 23 shown in FIG. 22.

As described above, according to this embodiment, since the highresistive component can easily be formed and still the resistance valuecan be controlled, the device destruction due to static electricity canbe prevented and the array inspection can be highly accurately performedas well. Further, since the destruction of static electricity in theunit assembly process can be dealt with after the panel is completed, anincrease in production volume owing to the improvement in fabricationyield and still highly reliable display can be provided.

As described above, according to the present invention, the liquidcrystal display provided with the electrostatic protection circuitsuperior in redundancy can be realized. Further, according to thepresent invention, the liquid crystal display provided with thesufficient protection function against static electricity in whichrelatively low voltage generates for a long period of time can berealized.

Furthermore, according to the present invention, the liquid crystaldisplay enabling to take measures against static electricity until thefinal stage of the substrate assembly process can be realized. Also,according to the present invention, the liquid crystal display in whichthe electrostatic protection element portion does not affect the size ofthe panel can be realized. Further, according to the present invention,the liquid crystal display having the electrostatic protection elementsection in which the element structure is simple and not unfavorable incontrolling the current can be realized.

1. An active matrix type liquid crystal display comprising: a switchingelement formed for each of a plurality of pixels decided by a pluralityof bus lines; a short ring connected to the plurality of bus lines; andan electrostatic protection element portion formed between each of theplurality of bus lines and the short ring, wherein the electrostaticprotection element portion comprises a thin film transistor having asource or a drain electrode connected to the bus lines and the drain orthe source electrode connected to the short ring, a conductive materialconnected to a gate electrode of the thin film transistor, a firstresistor connected to the conductive material for connecting the gateelectrode of the thin film transistor to the bus lines, and a secondresistor connected to the conductive material for connecting the gateelectrode of the thin film transistor to the short ring, and wherein thegate electrode of the thin film transistor is connected to theconductive material via capacitor.
 2. An active matrix type liquidcrystal display comprising: a switching element formed for each of aplurality of pixels decided by a plurality of bus lines; and anelectrostatic protection element portion formed between the adjacent buslines; wherein the electrostatic protection element portion comprises athin film transistor having a source or a drain electrode connected toone of the adjacent bus lines and the drain or the source electrodesconnected to the other of the bus lines, a conductive material connectedto a gate electrode of the thin film transistor, a first resistorconnected to the conductive material for connecting the gate electrodeof the thin film transistor to one of the bus lines, a second resistorconnected to the conductive material for connecting the gate electrodeof the thin film transistor to the other of the bus lines.
 3. an activematrix type liquid crystal display as set forth in claim 2 wherein thegate electrode of the thin film transistor is connected to theconductive material via capacitor.
 4. An active matrix type liquidcrystal display comprising: a switching element formed for each of aplurality of pixels decided by a plurality of bus lines; a short ringconnected to the plurality of bus lines; and an electrostatic protectionelement portion formed between each of the plurality of bus lines andthe short ring; wherein the electrostatic protection element portioncomprises a first thin film transistor having a source or a drainelectrode connected to the bus line and the drain or the sourceelectrode connected to the short ring, a conductive material connectedto a gate electrode of the first thin film transistor, a second thinfilm transistor having a source or a drain electrode connected to thebus line, the drain or the source electrode connected to the conductivematerial, and a gate electrode electrically floated, and a third thinfilm transistor having a source or a drain electrode connected to theshort ring, the drain or the source electrode connected to theconductive material, and a gate electrode electrically floated.
 5. Anactive matrix type liquid crystal display as set forth in claim 4wherein the gate electrode of the first transistor is connected to theconductive material.
 6. An active matrix type liquid crystal display asset forth in claim 4 wherein a channel length of at least one of thesecond and the third thin film transistors is shorter than a channellength of the first thin film transistor.
 7. An Active matrix typeliquid crystal display as set forth in claim 4 wherein the third thinfilm transistor is a common transistor connecting the gate electrodes ofthe plurality of the first thin film transistors to the short ring. 8.An active matrix type liquid crystal display comprising: a switchingelement formed for each of a plurality of pixels decided by a pluralityof bus lines; and an electrostatic protection element portion formedbetween the adjacent bus lines; wherein the electrostatic protectionelement portion comprises a first thin film transistor having a sourceor a drain electrode connected to one of the adjacent bus lines and thedrain or the source electrode connected to the other of the bus lines, aconductive material connected to a gate electrode of the first thin filmtransistor, a second thin film transistor having a source or a drainelectrode connected to one of the bus lines, the drain or the sourceelectrode connected to the conductive material, and a gate electrodeelectrically floated, and a third thin film transistor having a sourceor a drain electrode connected to the conductive material and a gateelectrode electrically floated.
 9. An active matrix type liquid crystaldisplay as set forth in claim 8 wherein the gate electrode of the firsttransistor is connected to the conductive material via capacitor.
 10. Anactive matrix type liquid crystal display as set forth in claim 8wherein a channel length of at least one of the second and the thirdthin film transistors is shorter than a channel length of the first thinfilm transistor.